The bacterial reaction center (RC) is the membrane protein responsible for the initial light induces electron and proton transfer reactions that convert light energy into chemical energy. A key component of this process is the bound quinone, QB, which undergoes the reaction QB + 2e- + 2H+ yields QBH2, coupling electron transfer with proton uptake from solution. This reaction is the first step in the proton pump that drives protons across the cell membrane using light energy. A major unanswered question involves the mechanisms for the transfer of electrons to QB. Recent results have indicated that the first and second electron transfers to QB are respectively dictated by protein dynamics and protonation equilibria. These mechanisms will be studied by changing residues near QB using site directed mutations, and by utilizing a novel technique for varying the driving force for electron transfer by quinone substitution to determine the rate limiting step for the electron transfer process. A second major question involves the mechanism and pathway for proton transfer to QB. Recent structural work has strongly suggested the role of internal water molecules in the pathway for proton transfer from the protein surface into the interior of the protein near QB. The role of these water molecules will be examined by mutation of protein residues associated with critical water molecules to block proton transfer to the QB site. The effect of these mutations on proton transfer will be monitored using assays for measuring the rate of proton transfer in the protein. These studies are particularly interesting due to recent advances in structural studies of other membrane proteins including Cytochrome Oxidase and the Cytochrome bc1 Complex that also catalyze proton-coupled electron processes. The studies on the RC can help to contribute to our understanding of this important class of proteins.